Radio communication systems are known to include a plurality of communication units that transceive information over communication resources via a plurality of base stations, or repeaters. Generally, radio communication systems also include a central controller, or communication resource allocator, that allocates the communication resources to the communication units. The communication units may be mobile radios, portable radios, or radiotelephones; whereas, the communication resources may be frequency carriers, pairs of frequency carriers, time slots, pairs of time slots, or combinations of time slots and frequency carriers, depending on the multiplexing scheme incorporated in the communication system. In a time division multiple access (TDMA) communication system, the communication resources comprise time slots or time slot pairs. When the communication resources comprise time slot pairs, each time slot pair includes an uplink (communication unit to base station) time slot and a downlink (base station to communication unit) time slot.
Utilization of time slots by communication units in a TDMA communication system is commonly accomplished using two known protocols, those being: slotted ALOHA and reservation ALOHA. With a slotted ALOHA protocol, each transmitting communication unit attempts to send its information in a randomly chosen uplink time slot to a base station, or repeater. When the selected uplink time slot is unoccupied, a communication unit's information is sent to the base station. However, when the random uplink time slot is currently occupied, a "collision" of information occurs and the transmitted information does not reach the base station. Accordingly, the information must be re-transmitted if the base station is to receive the information.
Due to the randomness of the slotted ALOHA protocol, slotted ALOHA is a very inefficient method for transmitting data packets. For single data packets (i.e., packets resident in one uplink time slot), the theoretical maximum throughput efficiency using slotted ALOHA is approximately thirty-seven percent. Thus, approximately one in every three transmission attempts is successful during maximum channel loading. The other sixty-three percent of the transmissions require re-transmission attempts. For data messages requiring multiple data packets (i.e., messages resident in multiple uplink time slots), each individual packet in the data message is transmitted using the slotted ALOHA protocol. Since each packet is subject to a possible "collision", throughput efficiency for large data messages is significantly worse than for a single packet transmission.
In contrast to slotted ALOHA, reservation ALOHA provides a more efficient procedure for transmitting long data messages by allocating uplink time slots to communication units on an exclusive use basis. In reservation ALOHA, the multitude of uplink time slots are divided into a plurality of uplink time frames. Thus, each uplink time frame includes a predetermined number of uplink time slots (e.g., five time slots per time frame). A communication unit transmits a small data packet to the communication resource allocator (also called a time slot allocator) in one uplink time slot of a particular uplink time frame via the slotted ALOHA protocol. The small data packet includes a request for the number of uplink time slots required to transmit the long data message. Upon receipt of the request packet, the time slot allocator allocates the requested number of uplink time slots by reserving one time slot in each succeeding uplink time frame to the requesting communication unit. Thus, the communication unit's transmission requires a substantially identical number of time frames as time slots to complete the transmission of the long data message. For example, when a communication unit requests four uplink time slots via the request packet, the time slot allocator allocates one time slot in each of the next four consecutive uplink time frames.
Although reservation ALOHA provides improved throughput efficiency for transmitting long data messages by employing time slot reservation, it inherently introduces lengthy transmission time delays due to its time slot allocation procedure. As described above, a multiple slot data message requires multiple uplink frame times to be transmitted. Thus, the time required to complete transmission of the multiple slot message is equivalent to the number of uplink time frames necessary to transmit the message one slot per frame. Depending on data message length and time frame period, this delay may be excessive. For example, with a time frame period of seventy-five milliseconds (fifteen milliseconds per time slot) and a data message length of four time slots, a transmission time of three hundred milliseconds is required.
Therefore, a need exists for a time slot allocation method that substantially reduces the transmission time delay associated with prior art allocation procedures and improves the transmission channel throughput efficiency.